The preparation of thin films of amorphous silicon, hereinafter referred to as .alpha.-Si, by the glow discharge decomposition of silane gas, SiH.sub.4, has been known for a number of years. (See, for example, R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Soc., 116, 77, 1969 and R. C. Chittick, J. N-Cryst. Solids, 3, 255, 1970). It is also known that the degree of conductivity and conductivity type of these thin films can be varied by doping with suitable elements in a manner analogous to that observed in crystalline semiconductors. (See, for example, W. E. Spear and P. G. LeComber, Solid State Commun., 17, 1193, 1975). Furthermore, it is widely recognized that the presence of atomic hydrogen plays a major role in the electrical and optical properties of these materials (see, for example, M. H. Brodsky, Thin Solid Films, 50, 57, 1978) and thus there is widespread current interest in the properties and uses of thin films of so-called "hydrogenated amorphous silicon", hereinafter referred to as .alpha.-Si(H).
Hydrogenated amorphous silicon, .alpha.-Si(H), is of increasing technological interest for applications such as photovoltaic devices, thin film electronic devices, and electrophotographic photoreceptors. The technological interest is largely due to a combination of the electrical and mechanical properties plus the fact that .alpha.-Si(H) can be readily fabricated into low cost, large area structures. The electrical properties are such that this material can be fabricated with either n-type or p-type conductivity over a range of some eight orders of magnitude. When prepared under optimum conditions, the photogeneration efficiency is near unity.
The field of electrophotography is one in which there is especially extensive current interest in the utilization of .alpha.-Si(H). To date, the art has disclosed a wide variety of photoconductive insulating elements, comprising thin films of intrinsic and/or doped .alpha.-Si(H), which are adapted for use in electrophotographic processes. (As used herein, the term "a doped .alpha.-Si(H) layer" refers to a layer of hydrogenated amorphous silicon that has been doped with one or more elements to a degree sufficient to render it either n-type or p-type). Included among the many patents describing photoconductive insulating elements containing layers of intrinsic and/or doped .alpha.-Si(H) are the following:
Kempter, U.S. Pat. No. 4,225,222, issued Sept. 30, 1980. PA0 Hirai et al, U.S. Pat. No. 4,265,991, issued May 5, 1981. PA0 Fukuda et al, U.S. Pat. No. 4,359,512, issued Nov. 16, 1982. PA0 Shimizu et al, U.S. Pat. No. 4,359,514, issued Nov. 16, 1982. PA0 Ishioka et al, U.S. Pat. No. 4,377,628, issued Mar. 22, 1983. PA0 Shimizu et al, U.S. Pat. No. 4,403,026, issued Sept. 6, 1983. PA0 Shimizu et al, U.S. Pat. No. 4,409,308, issued Oct. 11, 1983. PA0 Kanbe et al, U.S. Pat. No. 4,443,529, issued Apr. 17, 1984. PA0 Nakagawa et al, U.S. Pat. No. 4,461,819, issued July 24, 1984. PA0 (1) Relative to .alpha.-Si(H), the photogeneration efficiencies are extremely low. (For .alpha.-Ge(H) the photogeneration efficiencies are typically in the range of 10.sup.-4.) PA0 (2) The dark conductivity increases sharply with the incorporation of Ge. PA0 (3) Alloys containing Ge are prepared from gaseous GeH.sub.4 which is extremely toxic. PA0 (4) Due to the preferential attachment of H to Si, alloys of Si and Ge tend to be chemically inhomogeneous with respect to H. PA0 a support; PA0 a layer of hydrogenated amorphous silicon; PA0 a sensitizing layer comprising a phthalocyanine that serves as a spectral sensitizing agent; PA0 and a supersensitizing layer comprising an arylamine that serves as a chemical sensitizing agent.
Photoconductive elements that are useful in electrophotography comprise a conducting support bearing a layer of a photoconductive material which is insulating in the dark but which becomes conductive upon exposure to radiation. A common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and then imagewise expose it to radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface of the element and there dissipate the surface charge. This leaves behind a charge pattern in non-irradiated static image. This latent electrostatic image can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a developer which contains electroscopic marking particles. These particles are selectively attracted to, and deposit in, the charged areas or are repelled by the charged areas and selectively deposited in the uncharged areas. The pattern of marking particles can be fixed to the surface on which they are deposited or they can be transferred to another surface and fixed there.
For electrophotographic use, photoconductive elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers. Elements with multiple active layers (sometimes referred to as multi-active elements) have at least one charge-generating layer and at least one charge-transport layer. The charge-generating layer responds to radiation by generating mobile charge carriers and the charge-transport layer facilitates the migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge in exposed areas and form the electrostatic latent image.
Photovoltaic devices represent another very important area of technology in which there is great interest in the use of .alpha.-Si(H). These are devices which are useful for converting solar energy into electrical energy. In the past, the materials chiefly used in these devices have been inorganic crystalline semiconductors. However, such devices have proven to be very expensive to construct, due to the processing techniques necessary to fabricate the semiconductor layer.
A fundamental limitation of .alpha.-Si(H) is that the bandgap is approximately 1.70 eV. As a result, .alpha.-Si(H) shows very little photoconductivity in the near infrared region of the spectrum. This is a serious limitation for photovoltaic devices, since a significant fraction of the solar spectrum is in the near infrared region. For electrophotographic applications, the increasing interest in exposures derived from electronic light emitting devices, such as diode lasers or light emitting diodes, has placed similar requirements on infrared sensitivity. For these reasons, there is great interest in extending the action spectrum of .alpha.-Si(H) to longer wavelengths.
Unlike the chalcogenide glasses, .alpha.-Si(H) does not show a nonphotoconducting absorption edge. The action spectrum is symbatic with the absorption spectrum. As a result, the decrease in sensitivity with increasing wavelength is due to a decrease in absorption and not a decrease in the intrinsic photogeneration efficiency. Since the long wavelength absorption edge of .alpha.-Si(H) can be determined by structural disorder, there have been several attempts to extend the absorption edge by controlling the degree of disorder. In such a manner, the long wavelength edge of the action spectrum can be extended to 740 to 760 nm. This technique, however, requires a significant increase in the density of gap states which, in turn, sharply reduces the carrier lifetimes. In addition, the long wavelength shift is relatively small. For these reasons, this technique is of little practical significance.
An alternative method of spectral sensitization involves the formation of Si-containing alloys. By combining Si with other elements, it is possible to form alloys with lower bandgaps. Two such alloys have been described in the literature, one involving Sn (see, for example, I. Shimizu, Proceedings of the 11th International Conference on Amorphous and Liquid Semiconductors, Rome, Italy, 1985) and one based on Ge (see, for example, G. Nakamura et al, Jap. J. Appl. Phys. 20, 20-1, 291, 1981). Alloys of either Ge or Sn and Si can be prepared with the desired absorption spectra. In the case of the Sn alloys, the carrier lifetimes are extremely short. Further, these materials show no measurable photoconductivity and cannot be doped to either n- or p-type conductivity. Alloys of Ge and Si are photosensitive, however, these materials show other fundamental disadvantages. For example:
In view of these considerations, it is apparent that alloy formation is a sensitization technique with several fundamental limitations, particularly with respect to electrophotography.
It is toward the objective of providing an effective means for spectral sensitization of amorphous silicon photoconductive elements and especially of achieving a significant bathochromic shift, that is a shift to a region of longer wavelength, in the spectral sensitivity of such elements, that the present invention is directed.